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Range of lead free PCB finishes from The PCB Company

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With the growing demand for ROHS compliance, The PCB Company has increased the number of lead free PCB finishes it provides.

Lead free hot air level:

This finish utilises a compound primarily comprising of Tin. PCBs with this finish have a long shelf life (prior to soldering) and come at a similar price to the traditional Tin/Lead solder finish.

The finish is applied using the same technique and equipment as Tin/Lead solder, making it attractive for many PCB manufactures as they are able to convert their existing HAL equipment.

This finish has a higher melting temperature than traditional solder and hence subjects the PCB to higher thermal stresses during manufacture.

In addition, as the finish is levelled using a jet of hot air, the flatness of the finish is poor compared to other options.

Gold plate:

This finish consists of a thin layer of gold (Au) electroplated over a thicker layer of Nickel (Ni). The Nickel is electroplated directly over the bare copper.

This finish has the advantage of a long shelf life and due to the low oxidisation rate of the finish, can be left unprocessed to form electrical contact connections such as switches or circuit edge connectors.

The soldering process requires the solder compound to bond to the gold, unlike the Lead Free HAL finish where the solder compound can melt into the finish and mix on a molecular level.

As such a PCB with a gold plated finish requires accurate temperature profiling to ensure reliable solder joints across the entire PCB. While the finish is durable, the soldering process is susceptable to contamination from handling and the environment.

In addition, the finish contains a degree of Phosphor contamination within the gold due to processes involved during manufacture. When the PCB is subject to a heat process, the Phosphor tends to rise to the surface and detracts from the solderability of the finish.

Temperature profiling needs to take this into consideration when re-flow and wave soldering processes are utilised.

In particular the problem arises if a PCB needs to be exposed to more than one heat process during manufacture, for example soldering the SMD components through a re-flow process may cause solderability problems in subsequent soldering processes.

The finished assembly consists of solder joints comprising a copper PCB pad, a layer of electroplated nickel, then a layer of gold and then a layer of solder material.

The layers of different metals do suffer from an electrolysis affect, which in turn can cause the connection to corrode and fail over time. Though this is rare it is a documented phenomenon which needs to be considered particularly in high volume production.

ENIG (Electroless Nickel Immersion Gold):

This process consists of an Immersion Gold deposited over a layer of electroless Nickel. This finish has a long shelf life and is also suitable for Wire Bonding processes.

Like the gold plated finish, the gold contains a degree of phosphor contamination which rises to the surface when it is subject to a heat process. The gold layer is intentionally thin and is designed to protect the nickel from oxidisation.

The gold is chemically deposited and not plated, therefore during the soldering process a rapid dissolution of the gold coating occurs enabling the solder to bond directly to the Nickel.

OSP (Organic Solderability Preservative):

Also referred to as Flux Finish, the OSP is deposited directly into the bare copper and acts as a flux during the soldering process.

The finish has good solderability characteristic and provides a flat base for SMD components. As the solder paste bonds directly to the copper pad, long term reliability is high, which is why many multinational manufacturers prefer this finish.

Shelf life is classified as medium (6 months) and the finish is tolerant to small amounts of handling contamination, although handling will reduce shelf life.

Immersion silver:

This finish consists of very thin (0.15 to 0.55 micron) layer of almost pure silver deposited over the copper pads. A small amount of organic material is deposited within the silver to prevent tarnish and electromigration.

The finish is flat and has a medium shelf life. The solderability is relatively good compared to other finishes however is susceptable to tarnishing and contamination through handling and the environment.

If processing is carefully controlled, and boards are kept free of contamination and accurate temperature profiling is carried out, good consistent results are achieved with long term reliability.

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